Device for producing a composite component formed from carbon fibers coated with pyrolytic carbon

ABSTRACT

The invention relates to a method for producing a composite component and to a composite component, the composite component being formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy, a fiber composite being formed from the carbon fibers, a preform being formed from the fiber composite, the carbon fibers of the fiber composite being coated with pyrolytic carbon to form the preform, the preform being at least partially infiltrated with molten metal.

The invention relates to a method for producing a composite component and to a composite component, said composite component being formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy.

The metal-matrix composite materials used for forming composite components usually consist of a continuous metal matrix having a reinforcement formed from carbon fibers on the inside. Owing to their high stability and simultaneously low weight, these composite components are generally used in lightweight construction, such as in aircraft construction or astronautics. The carbon fibers reinforcing the metal matrix can be short-cut fibers or continuous fibers. The short-cut fibers can be added to and cast together with a metal melt, for example, in that case, however, inhomogeneities in the distribution of the fiber material can easily occur. For instance, depending on the shape of the mold or on the type of casting method used, an added amount of fibers can be unevenly distributed in an undesired manner within the composite component thus formed. This is also promoted in particular by the fact that carbon fibers have a substantially different density as compared to metal.

When casting or infiltrating carbon fibers with aluminum, for example, carbides or aluminum carbide may form, which leads to a dissolution of the carbon fibers if the carbon fibers stay in an aluminum melt for a longer duration, which, in turn, deteriorates the mechanical strength properties of the composite component thus formed.

Furthermore, the use of carbon-fiber-reinforced carbon as a material for forming a carbon composite is known. In a carbon-fiber-reinforced carbon, carbon fibers are first soaked with a resin, for example, and the resin is subsequently pyrolized. However, a preform formed in this way can no longer be completely infiltrated with a metal because the gaps between the carbon fibers will have been almost completely filled with pyrocarbon, which then forms the matrix of the composite material, Nevertheless, it is possible to coat a carbon-fiber-reinforced carbon or a composite component of this kind with a metal, in which case, however, the mechanical strength properties of a metal-matrix composite component cannot achieved.

Therefore, the object of the present invention is to propose a method for producing a composite component and a composite component produced by said method that has improved strength properties.

This object is attained by a method having the features of claim 1 and by a composite component having the features of claim 21.

In the method according to the invention for producing a composite component, the composite component is formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy, a fiber composite being formed from the carbon fibers, a preform being formed from the fiber composite, the carbon fibers of the fiber composite being coated with pyrolytic carbon to form the preform, and the preform being at least partially infiltrated with molten metal.

The mechanical strength properties of the composite component can thus be improved by first forming a fiber composite from the carbon fibers, said fiber composite having a defined geometry. Undesired inhomogeneities of the carbon fibers in the composite component can be avoided in this way. From the fiber composite, a dimensionally stable preform is formed by coating the carbon fibers of the fiber composite with pyrolytic carbon. Thus, the carbon fibers are completely surrounded by the pyrolytic carbon, the carbon fibers being connected to one another at each of their mutual contact points by the coating made of pyrolytic carbon. Since the carbon fibers are coated with a relatively thin layer of pyrolytic carbon, a gap remains between the carbon fibers, which ensures sufficient porosity of the preform suitable for infiltration with a molten metal. The dimensionally stable preform can thus be infiltrated with the molten metal without dissolving or changing the geometric shape of the preform. Also, the pyrolytic carbon forms a protective layer on the carbon fibers, which prevents a formation of carbides and thus a dissolution of the carbon fibers. Moreover, the coating with pyrolytic carbon leads to improved wettability of the carbon fibers. Overall, a geometrical orientation of the carbon fibers can be fixed in this way, the carbon fibers themselves being maintained and adhering to one another. Thus, a composite component produced by the method according to the invention exhibits improved mechanical strength properties as compared to a conventional composite component, also with respect to a comparable component weight.

In the course of the method, the pyrolytic carbon can be vapor-deposited on the carbon fibers. In this way, it becomes possible to coat the carbon fibers with a comparatively thin layer of pyrolytic carbon. Furthermore, vapor deposition allows a particularly easy adjustment of a layer thickness. Also, it is possible to coat fiber composites having almost any kind of geometry and carbon fiber density with pyrolytic carbon because the gas in question can easily permeate the fiber composite.

Preferably, the pyrolytic carbon can be formed as a deposit produced on the carbon fibers by means of a CVD method or a CVI method. In this way, coating of the carbon fibers with pyrolytic carbon can be can led out in a particularly simple manner. It is also possible to envisage multiple treatment steps in which the fiber composite is deposition-coated with pyrolytic carbon or with so-called glassy carbon by means of the CVD and/or CVI method.

In an alternative variation of the method, the coating with pyrolytic carbon on the carbon fibers can be formed by pyrolizing a thin resin or pitch layer on the carbon fibers. In case of vapor-coating, a component size is determined by a wall thickness of the fiber composite or composite component because of a permeation of the fiber composite with vapor and a size of a reactor chamber for vapor deposition usually limited by the constraints of the process. Owing to the coating with the thin, low-viscosity, liquid resin layer, it becomes possible to produce composite components of almost any size. Furthermore, pyrolysis can take place in a conventional pyrolysis furnace, whose size is not limited by the constraints of the process.

Preferably, a thickness of the resin or pitch layer can be formed smaller than a thickness of the carbon fibers. This may be required in order to ensure sufficient porosity of the preform for infiltration. In particular, gaps between the carbon fibers will consequently not be completely filled with resin, or pyrocarbon, so that interconnected gaps are formed. Preferably, the thickness of the resin layer can be formed 50 percent smaller and particularly preferably 80 percent smaller than a thickness of the carbon fibers.

The resin layer can be formed easily by soaking the fiber composite in a highly diluted phenolic resin solution, e.g., phenolic resin diluted with ethanol or acetone. In this way, it is possible that liquid resin can completely permeate the fiber composite irrespective of a wall thickness. Resin soaking can also take place in a vacuum atmosphere. In a subsequent pyrolysis by means of hardening and coking or pyrolysis at 1000 to 2000° C., for example, a coating of the carbon fibers made of pyrolytic glassy carbon can be produced in this way.

Advantageously, the coated carbon fibers can be provided with another coating made of silicon carbide. In this way, it becomes possible to change the mechanical properties of the composite component in a desired manner where appropriate and, in case aluminum is used as a matrix material, for example, to avoid an undesired chemical reaction of the aluminum during infiltration. Also, carbon fibers coated in this manner or a network or fabric of carbon fibers forming the preform have an increased stiffness, which is particularly advantageous for the subsequent method step of infiltration.

Furthermore, it may be envisaged to form an at least partially unidirectional orientation of the carbon fibers of the fiber composite. For example, continuous fibers can be brought into a desired geometric form by winding or any other technique. In principle, however, it is also possible to use short-cut fibers without a specific spatial orientation for coating with pyrolytic carbon. The short-cut fibers can be present in the form of a fiber mat or fleece, and the fiber mat or fleece itself can be used for geometric forming of the preform. Preferably, however, fiber tissue mats or filament yarns can be used to form a potentially multilayer fiber composite.

Furthermore, it may be envisaged to compress the fiber composite prior to forming the preform by coating. In this way, it can be achieved that the carbon fibers are in tight contact with one another and a percentage by volume of carbon fibers in the composite component is substantially increased. Adjuvants can be added to the fiber composite during compression, which make the fiber composite or the carbon fibers adhere to one another and thus fix them for the time being without substantially reducing a porosity of the fiber composite.

In a particularly preferred manner, the fiber composite can be formed as a spatially oriented support structure of the composite component, said support structure being adjusted to a load case of the composite component. Ideally, the fiber composite can be arranged in the composite component in such a manner or the carbon fibers can be oriented in the composite component in such a manner that forces or stresses within the composite component run substantially in the direction of the longitudinal extension of the carbon fibers when the composite component is being used as intended so as to achieve maximum mechanical strength of the composite component. A composite component that is primarily subjected to tensile loads, for example, may have a support structure of carbon fibers that are spatially oriented in the direction of the tensile stresses. Depending on the intended load case of the composite component, the carbon fibers of the fiber composite can also be arranged in a combination of different spatial orientations.

Composite components having particularly complex geometric forms become especially easy to produce if a support structure of the composite component is formed by a plurality of preforms. For instance, individual preforms can be formed which are put together to form a support structure of the composite component. For example, the preforms can engage into one another in a form-fitting manner or also be arranged independently of one another within the composite component. In this way, it becomes possible to produce composite components having almost any kind of geometry because potential geometric constraints for forming the fiber composite from carbon fibers no longer have to be necessarily considered. It is also contemplated to mechanically process a preform prior to infiltration, such as by machining, in order to arrive at the desired geometric form of the support structure or preform. This becomes possible in particular because the preform is dimensionally stable owing to the coating with pyrolytic carbon.

The preform can be infiltrated with aluminum, titanium, magnesium, copper or an alloy of one of these metals. In principal, any metal and any alloy that has a melting point which does not cause the pyrolytic carbon coating of the carbon fibers to dissolve is suitable for infiltration, Aluminum in particular is especially suitable because it is light and easy to process as a matrix material for lightweight-construction composite components.

It is substantial for the production of a composite component that the preform can be formed with an open pore structure. In this way, it becomes possible to easily infiltrate the preform with the molten metal. At least partially closed pore structures prevent the preform from being completely infiltrated With metal and result in a formation of cavities, which have a negative impact on the mechanical strength of the composite component.

Accordingly, the preform can be completely infiltrated with molten metal. Thus, an interconnected matrix made of metal can be achieved, which completely fills substantially all gaps of the fiber composite of the carbon fibers of the preform. In a particularly simple variation of the method, it may be envisaged to infiltrate the preform with the molten metal by simply immersing it in a metal melt. The preform can stay in the metal melt for a relatively long time because dissolution of the carbon fibers or carbide formation is prevented by the coating made of pyrolytic carbon. It is also possible to infiltrate the preform with molten metal for several times in order to achieve complete infiltration.

In another embodiment of the method, it is possible to arrange the preform in a casting mold, In this way, a composite component having a desired geometric form can be cast, and the preform can then be infiltrated with molten metal during the casting process. For example, one or more preforms can be placed into a casting mold in the manner of a core, wherein the preform can fill the casting mold completely or only in part. Furthermore, the preform can be arranged in the casting mold in such a manner that the preform is infiltrated with metal only in sections, which means that a section of the composite component thus obtained can consist solely of coated carbon fibers without a metal matrix, while another section of the composite component comprises coated carbon fibers with as metal matrix. Moreover, the composite component can also have a section that is formed solely by the matrix material or the metal. In this way, it becomes possible to produce composite components that have component sections which are adjusted to load cases or certain applications.

An infiltration of the preform can take place by means of pressure die casting, squeeze casting or vacuum casting. For example, composite is components that have a particularly accurate size can be produced by pressure die casting. Through vacuum casting, it is easy to achieve complete infiltration of the preform with metal, Owing to its dimensional stability, the preform can be integrated particularly easily into the aforementioned casting methods for producing the composite component.

In one embodiment of the method, the composite component can be formed in such a manner that it has a metal content of more than 50 percent by volume. This is particularly advantageous if a higher metal content has a particularly favorable effect on the properties of the Composite component corresponding to its intended use.

In another embodiment of the method, the composite component can be formed in such a manner that it has a carbon fiber content of more than 50 percent by volume, This may be particularly advantageous if an intended use of the composite component is benefited by a particularly high carbon fiber content in the composite component.

It may also be advantageous if the composite component is formed in such a manner that the carbon fibers are distributed homogeneously within the composite component. In that case, the composite component consists of a homogenous metal-matrix composite material having regular material properties except for a fiber orientation.

However, the composite component can also be formed in such a manner that the carbon fibers are distributed heterogeneously within the composite component. This means that sections of the composite component can have a higher or lower content of carbon fibers. Owing to the dimensional stability of the preform, it is possible to specifically set or predetermine the content of carbon fibers within the composite component and also the spatial orientation of the carbon fibers in order to influence the mechanical properties of the composite component.

The composite component according to the invention is formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy, a fiber composite being formed from the carbon fibers, a preform being formed from the fiber composite, the carbon fibers of the fiber composite being coated with pyrolytic carbon to form the preform, the preform being at least partially infiltrated with molten metal. The composite component according to the invention can be produced by means of the method according to the invention. Regarding the advantageous effects of the composite component according to the invention, reference is made to the description of advantages of the method according to the invention. Other embodiments of the composite component become apparent from the dependent claims back-referenced to method claim 1. 

1. A method for producing a composite component, the composite component being formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy, said method comprising: forming a fiber composite from carbon fibers; coating carbon fibers of the fiber composite with pyrolytic carbon to form a preform; and at least partially infiltrating the preform with molten metal.
 2. The method according to claim 1, in which the pyrolytic carbon is vapor-deposited on the carbon fibers.
 3. The method according to claim 2, in which the pyrolytic carbon is formed as a deposit produced on the carbon fibers by CVD method or a CVI method.
 4. The method according to claim 1, in which the pyrolytic carbon on the carbon fibers is formed by pyrolizing a thin resin or pitch layer on the carbon fibers.
 5. The method according to claim 4, in which a thickness of the resin or pitch layer is formed smaller than a thickness of the carbon fibers,
 6. The method according to claim 4, in which the resin layer is formed by soaking the fiber composite in as diluted phenolic resin solution.
 7. The method according to claim 1, in the coated carbon fibers are provided with another coating made of silicon carbide.
 8. The method according to claim 1, in which an at least partially unidirectional orientation of the carbon fibers of the fiber composite is formed.
 9. The method according to claim 1, in which the fiber composite is compressed.
 10. The method according to claim 1, in which the fiber composite is formed as a spatially oriented support structure of the composite component, said support structure being adjusted to a load case of the composite component.
 11. The method according to claim 1, in which a support structure of the composite component is formed by a plurality of preforms.
 12. The method according to claim 1, in which the infiltration is performed with aluminum, titanium, magnesium, copper or an alloy of one of these metals.
 13. The method according to claim 1, in which the preform is formed with an open pore structure.
 14. The method according to claim 1, in which the preform is completely infiltrated with molten metal.
 15. The method according to claim 1, in which the preform is arranged in a casting mold.
 16. The method according to claim 15, in which the infiltration is performed by means of pressure die casting, squeeze casting or vacuum casting.
 17. The method according to claim 1, in which the composite component is formed in such a manner that it has a metal content of more than 50 percent by volume.
 18. The method according to claim 1, in which the composite component is formed in such a manner that it has a carbon fiber content of more than 50 percent by volume.
 19. The method according to claim 1, in which the composite component is formed in such a manner that the carbon fibers are distributed homogenously within the composite component.
 20. The method according to claim 1, in which the composite component is formed in such a manner that the carbon fibers are distributed heterogeneously within the composite component.
 21. A composite component formed from a metal-matrix composite material made of carbon fibers and a metal or a metal alloy, said composite component comprising: a preform formed from a fiber composite formed from carbon fibers, the carbon fibers of the fiber composite being coated with pyrolytic carbon, the preform being at least partially infiltrated with molten metal. 